The instant invention relates to heating apparatus for use in heating articles and more particularly to an apparatus for heating articles utilizing a vaporized heat transfer medium.
Processes wherein articles are heated utilizing vaporized heat transfer media have been generally known for a number of years. More specifically, processes of this type have been known and have generally been found to be effective for use in various reflow processes wherein soldering or brazing compounds are heated to temperatures above the melting points thereof to form soldered or brazed connections. In particular, processes of this type have been found to be highly effective for use in reflow soldering operations wherein solder is melted to form soldered joints between electrical components on printed circuit boards and the like. In this specific application, perfluorinated liquids have been widely used and have been found to be highly effective heat transfer media.
While heating apparatus which use vaporized heat transfer media, such as perfluorinated compounds, have been found to be highly effective for use in reflow soldering operations, they have generally been found to have excessively high operating costs, primarily due to the fact that they have been prone to relatively high vapor losses. In this connection, the heretofore available apparatus for carrying out processes of this type have generally comprised heating vessels containing quantities of vaporized heat transfer media for heating articles as they are passed therethrough. Apparatus of this type have generally been adapted for continuous operations, and hence they have generally had normally open inlet and outlet openings to enable articles to be passed therethrough. It has been found that substantial quantities of vaporized heat transfer media are often lost through the open inlet and outlet openings of the heating vessels of apparatus of this type as articles are passed into or removed from the vessels. In this regard, it has been found that due to the extremely high cost of certain types of heat transfer media, such as perfluorinated liquids, vapor losses can represent significant cost factors in solder reflow processes. It has also been found that the vapors which are lost in heat transfer processes of this type can be environmentally harmful.
In addition to the problem of vapor losses, it has been found that many of the heretofore available vapor heating apparatus have been prone to accumulating moisture in the heating vessels thereof, and it has been further found that accumulated moisture can detract from the effectiveness of most vaporized heat transfer media, particularly those generated from perfluorinated liquids. In this connection, many of the heretofore available heating apparatus have included cooling coils in the main vessels thereof for containing the vapor blankets thereof, and it has been found that the cooling coils in heating apparatus of this type can cause moisture condensation. Further, it has been found that the configurations of the heating vessels of most of the heretofore available apparatus of this type inherently cause moisture to be accumulated therein rather than passing outwardly through the inlet or outlet openings thereof.
Heating apparatus representing the closest prior art to the subject invention of which the applicant is aware are disclosed in the PFAHL JR. et al, U.S. Pat. Nos. 3,866,307; CHU et al, 3,904,102; PFAHL, JR., 3,947,240; SHARVINKO et al, 4,022,371; CHU et al, 4,032,033; CHU et al, 4,055,217; SPIGARELLI, 4,077,467; CHU et al, 4,090,843; and AMMANN et al, 4,115,601. However, while some of the devices disclosed in these references have generally addressed the problem of reducing the losses of heat transfer media, none of them have effectively solved this problem. They have also not solved the problem of moisture accumulation. Accordingly, these references are believed to be only of general interest with respect to the instant invention, and the apparatus of the instant invention is believed to represent a significant improvement over the devices disclosed in these references.
The instant invention provides an effective apparatus for heating articles utilizing a vaporized heat transfer medium wherein the losses of the vaporized heat transfer medium are essentially eliminated. Specifically, the apparatus of the instant invention comprises a vessel having an exhaust opening therein and having inlet and outlet openings therein which are open to the atmosphere. The apparatus further comprises a quantity of vaporizable liquid heat transfer medium in the vessel, the heat transfer medium being of a type which is vaporizable to produce a vapor having a density greater than that of air, and means for heating the heat transfer medium to produce a vapor blanket of vaporized heat transfer medium in the vessel. The apparatus further comprises means for controlling the vapor blanket to maintain the upper extremity thereof at a predetermined level, and the apparatus is constructed so that portions of the inlet opening, the outlet opening and the exhaust opening are above the predetermined level of the vapor blanket. Still further the apparatus comprises exhaust means for applying a partial vacuum to the vessel through the exhaust opening in order to slowly withdraw air and any of the heat transfer medium intermixed therewith from the vessel, and transport means for transporting articles through the vessel so that they pass inwardly through the inlet opening, through the vapor blanket and then outwardly through the outlet opening. The preferred embodiment of the apparatus further comprises recovery means for recovering any of the heat transfer medium intermixed with the air which is withdrawn from the vessel through the exhaust opening, the recovery means preferably comprising spaced, coaxial inner and outer, substantially upright columns, a plurality of contact pins between the inner and outer columns and means for supplying water to the inner column so that the water spills out the upper end thereof and passes between the inner and outer columns. In this embodiment of the apparatus, the exhaust means is operative for drawing air and any of the heat transfer medium intermixed therewith upwardly between the inner and outer columns to intermix the heat transfer medium with the water while the air is drawn upwardly from between the columns by the exhaust means. The apparatus preferably further includes cooling means in the vessel above the predetermined level of the vapor blanket for cooling the portions of the vessel which are above the vapor blanket, and the vessel is preferably constructed so that it includes an upper portion which is disposed above both the inlet opening and the outlet opening. The exhaust opening is preferably disposed in the upper portion of the vessel, and the cooling means is preferably operative for cooling the upper portion. Still further, the apparatus preferably includes inlet and outlet tunnels which communicate with the inlet and outlet openings, respectively, and the inlet and outlet tunnels are preferably open to the atmosphere, although each preferably includes a segment which is entirely above the predetermined level of the vapor blanket in the vessel.
It has been found that the heating apparatus of the instant invention effectively solves the problem of preventing the loss of heat transfer medium. Specifically, because the inlet opening, the outlet opening, and the exhaust opening are all at least partially above the predetermined level of the upper extremity of the vapor blanket, it is possible to produce a positive inward flow of air through the inlet opening and the outlet opening. Accordingly, when a partial vacuum is applied to the vessel by the exhaust means, air flows inwardly through the inlet opening and the outlet opening to prevent vaporized heat transfer medium from passing outwardly through these openings. Further, because the apparatus preferably includes recovery means for recovering any of the heat transfer medium entrained in the air which is withdrawn from the vessel through the exhaust opening, it is possible to recover virtually all of the heat transfer medium which is withdrawn from the vessel and to exhaust air which is essentially pure. Still further, because there is a constant flow of air through the apparatus, any moisture which is contained in the air is also withdrawn so that moisture cannot accumulate or condense in the main vessel of the apparatus.
As a result, it is a primary object of the instant invention to provide an apparatus for heating articles utilizing a vaporized heat transfer medium wherein the loss of heat transfer medium is minimized.
Another object of the instant invention is to provide an apparatus for heating articles utilizing a vaporized heat transfer medium wherein moisture buildup is eliminated.
An even further object of the instant invention is to provide an apparatus for soldering electrical components on printed circuit boards utilizing a vaporized heat transfer medium wherein the loss of heat transfer medium is minimized.
A still further object of the instant invention is to provide an apparatus for heating articles utilizing a vaporized heat transfer medium comprising a vessel having an inlet opening, an outlet opening, and an exhaust opening and a vapor blanket in the vessel, wherein air is constantly drawn inwardly through the inlet and outlet openings and withdrawn through the exhaust opening.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.